A common problem in modern ad-hoc wireless communications networks is that individual nodes need to recognize each other's existence, and possibly each other's locations, to be able to join together to form a network. In military communications systems fast and covert new node identification and recognition means can help prevent friendly fire incidents. Once a network is established, new nodes often need to join the existing network. The nodes need a way to do this without compromising their own security, or the security of the network they are joining. Additionally, an established network typically uses a method of discovering the existence of another disjoint network that has migrated into communication range, so that a cross-link can be established between the networks to form a larger network. This process of nodes “discovering” each other is called node discovery.
There are many ways that node discovery can be performed. A good node discovery scheme for an encrypted or secret communications network has a number of properties, including permitting very fast and reliable network entry, being covert, secure and jam proof, as well as having a range that exceeds the network itself. One procedure used to accommodate these desired properties is to spread the carrier signal to form a spread spectrum signal.
Spread spectrum techniques have proven useful in a variety of communications applications, including cellular telephones, wireless networks, and military communications. One advantage provided by spread spectrum techniques is the ability to build a transmitter which is difficult for an unauthorized user to detect.
Wireless spread spectrum systems operate by using a relatively large amount of spectrum bandwidth to communicate their signals. The large bandwidth is consumed by spread spectrum encoding the message data using a pseudonoise code. The two most common types of spread spectrum transmission are frequency hopping, where the pseudonoise code is used to pseudo randomly change the transmission frequency on a periodic basis, and direct sequence, where the pseudonoise code is used to modulate the transmit signal at a relatively high rate compared to the underlying message data rate.
In order to detect a spread spectrum transmission, it is generally necessary to know the pseudonoise code beforehand. Furthermore, to extract the message data, it is generally necessary to know the timing of the pseudonoise code. For example, in a direct sequence system, this can be accomplished by knowing the code frequency, also known as the chip rate (rate at which the pseudonoise code advances through its sequence), and the starting time of the pseudonoise code (sometimes referred to as the phase of the code). A signal for which the spread spectrum receiver knows the pseudonoise code, pseudonoise code phase, and pseudonoise code frequency can be referred to as a synchronized signal.
Achieving synchronization with a spread spectrum signal can be difficult, in part due to high pseudonoise code rate (frequency). For example, a relatively low message data rate of 1,000 bits per second might be spread spectrum encoded with a relatively high pseudonoise code rate of 10,000,000 chips per second, where a bit of the pseudonoise code is referred to as a chip. In this example, the ratio of 10,000,000/1,000=10,000 is the processing gain. A spread spectrum receiver for this signal will need to synchronize to the high pseudonoise code rate being used by the transmitter, and hence the spread spectrum receiver requires a factor of 10,000 higher synchronization accuracy than a non spread spectrum system. The difficulty of achieving this synchronization increases as the processing gain increases.
In order to limit the difficulty of synchronizing spread spectrum systems, various techniques have been used. These techniques include the use of very stable oscillators to generate the carrier frequency on which the transmission is centered, the use of very stable clocks to generate the pseudonoise code, and the transmission of special pilot signals or long preambles of known data to aid receiver in synchronization.
Another property of spread spectrum systems is a generally low probability of detection by a user lacking knowledge of the pseudonoise code. This is because the transmitter power of the spread spectrum signal is spread out over a relatively large portion of radio spectrum. By using a high processing gain, it is possible to sufficiently spread the transmitter power out so that the resulting transmission spectral power density is below the noise level within the environment. In general, it is more difficult to detect a spread spectrum signal without knowledge of the pseudonoise code as the processing gain is increased, making the use of high processing gain desirable.
Disrupting transmissions of spread spectrum signals is technically possible. For example, a foe may be able to transmit noise or some other type of signal over the same bandwidth as the spread spectrum signal. The noise may be broadcast at a sufficiently high power that the signal to noise ratio of the transmitted signal is low enough that it becomes difficult to detect the signal out of the noise. However, at sufficiently high processing gains the spread spectrum signal is broadcast over a relatively wide signal bandwidth. The amount of power that would be necessary to broadcast over substantially all of the bandwidth to disrupt the spread spectrum power becomes unfeasible. Thus, a foe may look for other ways to disrupt a spread spectrum signal having a high processing gain.